This invention relates to the art of materials science and, more particularly, to nonmetallic materials and powder metallurgy.
Ceramic materials have certain outstanding properties, such as high temperature strength, corrosion resistance, low density, and low thermal expansion, which make them attractive materials for high temperature applications. However, ceramics differ from metals in one very important aspect: they are brittle, that is, upon loading, they do not deform before fracturing. This lack of a stress-relieving characteristic, which also causes ceramics to have low tolerance for flaws, is a major drawback to using them in high temperature structural applications.
There is a class of materials which offers the advantages of ceramics and certain of the beneficial mechanical characteristics of metals. These materials are intermetallics, which at high temperatures have the desirable properties of ceramics, but also behave mechanically like metals in that they show yielding and stress-relieving characteristics.
Molybdenum disilicide (MoSi.sub.2) is an intermetallic compound which has potential for structural use in oxidizing environments at high temperatures. It has a melting point of 2030 C. and its oxidation resistance at high temperature is very good. Mechanically, MoSi.sub.2 behaves as a metal at high temperatures, since it undergoes a brittle-to-ductile transition at about 900-1000 C. Thus, MoSi.sub.2 has a stress relieving characteristic at high temperatures. The major problems impeding the use of MoSi.sub.2 as a high temperature structural material with potential use temperatures in the range of 1200-1800 C. are its relatively low strength at high temperatures and its brittleness or lack of fracture toughness at low temperatures. Fracture toughness may be defined as resistance to fracture. At low temperatures, strength of MoSi.sub.2 is limited by brittle fracture, while at high temperatures, it is limited by plastic deformation or creep. For this material to be a viable structural material at high temperatures, both its elevated temperature strength and its room temperature fracture toughness must be improved.
Zirconium oxide (zirconia, ZrO.sub.2) is a ceramic which possesses high strength and high fracture toughness at room temperatures, but not at high temperatures. Composites comprised of MoSi.sub.2 and ZrO.sub.2 have high fracture toughness at room temperatures due to the ZrO.sub.2 and at high temperatures due to the MoSi.sub.2. Pure zirconia exists in a tetragonal crystalline state at high temperatures and in a monoclinic state at low temperatures. As tetragonal zirconia is cooled through its transformation temperature, there is a volume change which is sufficient to exceed elastic and fracture limits and can only be accommodated by cracking. Thus, fabrication of large components of pure zirconia is not possible because they develop cracks upon cooling. However, this volume expansion of the tetragonal to monoclinic transformation can be used to improve fracture toughness by combining zirconia with other materials. U.S. Pat. No. 5,063,182 (Petrovic et al., issued Nov. 5, 1991) teaches composites of zirconia and molybdenum disilicide.
The properties of zirconia can be modified by the addition of crystallographic stabilizing agents. These stabilizing agents include yttrium oxide (Y.sub.2 O.sub.3), magnesium oxide (MgO), calcium oxide (CaO), and cerium oxide (CeO.sub.2). A mixture of zirconia and stabilizing agent may be characterized as partially stabilized or fully stabilized. Partially stabilized zirconia (PSZ) remains in the tetragonal state upon cooling but will partially transform to the monoclinic state under certain circumstances. Fully stabilized zirconia (FSZ) is in the cubic crystalline state at high temperatures and remains so as it is cooled. The amounts of stabilizing agent to obtain partial stabilization and full stabilization varies with the stabilization agent used and can be determined from a phase diagram for zirconia and the stabilizing agent.
Information on zirconia is available in a publication by Magnesium Elektron LTD. of Twickenham, England entitled "An Introduction To Zirconia; Zirconia And Zirconia Ceramics," which was written by R. Stevens of the University of Leeds.
Silicon carbide (SIC) whiskers have been used to reinforce MoSi.sub.2 and a composite of SiC whiskers in MoSi.sub.2 exhibits improved high temperature strength, as compared to pure MoSi.sub.2. U.S. Pat. Nos. 4,927,792 (Petrovic et al., issued May 22, 1990) and 5,000,896 (Petrovic et al., issued Mar. 19, 1991) teach compositions of MoSi.sub.2 and SiC whiskers. Whiskers of two types are available, those made by a vapor-liquid-solid process (VLS whiskers) and those made by a vapor-solid process (VS whiskers). In accordance with the present invention, the addition of SiC, in whisker or powder form, to MoSi.sub.2 reinforced with zirconia provides the resulting composite material with a further improvement in high temperature strength.
Examples of immediate applications for the inventive materials are engine turbocharger rotors, turbine engine hot section components, high temperature furnace heating elements, and adiabatic diesel engines, which do not need a cooling system. Because the room temperature electrical conductivity of MoSi.sub.2 is relatively high, it may be possible to use electrodischarge machining of the inventive composites. This method of machining is significantly less expensive than the diamond machining process which is presently used for zirconia objects. Also, though zirconia will not couple to 2.45 GHz microwave radiation at room temperature, it is expected that the inventive composites will do so, so that microwave processing can be used in their manufacture.